1. Field of the Invention
The present invention relates to tunable light source apparatuses used in optical fiber communication, light source wavelength adjustment methods, and light source wavelength adjustment programs, in particular, to a tunable light source apparatus used in an optical communication of WDM (Wavelength Division Multiplexing) transmission system, a light source wavelength adjustment method, and a light source wavelength adjustment program.
2. Related Art
Recently, in the field of optical communication, the WDM transmission system of converting a plurality of data signals to light signals of different wavelengths, and multiplexing the plurality of light signals of different wavelengths to be transmitted in one optical fiber thereby realizing a large capacity optical transmission is widely put to practical use. Further, practical application of the DWDM (Dense Wavelength Division Multiplexing) transmission system, which can realize denser multiplexing than the WDM technique, is advancing.
In the optical communication system of the WDM transmission system, the transmission light wavelength is set on a frequency grid (ITU-Grid) standardized by the ITU (International Telecommunication Union). Thus, for every wavelength on the ITU-Grid, a corresponding light source is required respectively, and furthermore, since the ITU-Grid frequency spacing is set to be small in the DWDM, the number of settable wavelength increases, and greater number of light sources are required. In order to solve such disadvantage, practical application of a tunable light source apparatus in which the output wavelength can be freely controlled is advancing.
In order to realize a reliable optical communication, the tunable light source apparatus needs to set the frequency of the output light on the ITU-Grid, and continuously lock the frequency of the output light within a frequency range of about 1 GHz from such set frequency. A multiple-optical resonator type tunable light source apparatus serving as the tunable light source apparatus described above is disclosed in Japanese Laid-Open Patent Publication No. 2006-196554 (Patent Document 1).
The tunable light source apparatus of Patent Document 1 has a configuration in which laser oscillation occurs at a wavelength at which all three transmission resonance frequencies of three ring resonators coincide, where the desired laser oscillation frequency is output by adjusting the input power to a TO (Thermo-Optic) phase shifter arranged in each ring resonator.
In such tunable light source apparatus, the filter loss becomes the smallest, and the stability of the wavelength and the power tolerance of the TO can be maximized when the resonance frequencies of the three ring resonators are exactly coinciding on the oscillation frequency.
However, in the multi-purpose tunable light source apparatus, the laser oscillation wavelength fluctuates and deviates from the ITU-Grid due to change in outside temperature and variation in refractive index of the light waveguide portion. Particularly, since the tunable light source apparatus of external resonator type, such as PLC (Planar Lightwave Circuit) type, has a structure in which the oscillation frequency can be freely changed, and thus has a characteristic in that the wavelength tends to easily vary inherently. For this reason, in order to maintain the stability of the laser wavelength over a long period of time, the shift of central wavelength needs to be detected and corrected with various methods.
In the tunable light source disclosed in patent document 1, frequency ripples having a period of various sizes are generated due to the presence of various minor reflections such as internal reflection of an SOA (Semiconductor Optical Amplifier), PLC/SOA connecting point reflection and the like. Since the frequency and the intensity of the ripple are changed depending on gain current, environment temperature, and the like, the condition in which the exit light level and the SMSR (Sub-Mode Suppression Ratio) become a maximum also changes.
Due to the influence of such ripples, a state in which the central frequencies in the three ring resonators do not coincide arises even if the exit light level is a maximum, and whether or not the three central frequencies are exactly coinciding on the oscillation frequency is difficult to be determined only from the characteristics of the output exit laser beam.
In the multiple-optical resonator disclosed in patent document 1, when laser oscillation starts at a certain wavelength, the gain is concentrated thereby other sub-oscillation modes are suppressed, and thus laser oscillation similarly occurs even if the central frequencies in the three ring resonators are slightly shifted, and whether or not the three central frequencies are accurately coinciding on the oscillation frequency is difficult to be determined only from the characteristics of the exit light.
A mode gain difference, which is the transmission loss difference between one oscillating wavelength channel and an adjacent oscillating channel, in a spectrum of a single mode laser from the PLC multiple-optical resonator disclosed in patent document 1 takes a maximum value when all three central frequencies in the three ring resonators exactly coincide. Disadvantages such as unexpected wavelength skipping etc. arise unless the assumed mode gain difference is obtained. If the wavelength of the tunable light source automatically switches to another wavelength due to wavelength skipping, the communication of the relevant wavelength becomes disconnected, and furthermore, the communication of another wavelength channel also becomes disconnected.
In the multiple-optical resonator type tunable light source apparatus of Patent Document 1, in order to increase the mode gain difference, a method of increasing the finesse of the optical resonator filter may be adopted. The frequency characteristics of the optical resonator filter becomes of narrower band and the gain difference also becomes larger as the number of average turns of the ring resonator is increased. If the average number of turns increases, the propagation loss accumulates and the insertion loss increases as it passes through the ring resonator that many times, and thus a trade off state such that the output of the light source becomes difficult to be obtained arises. Thus, the design is required to be made at a necessity minimum finesse, and it is important to reliably coincide the central frequencies of the three ring resonators in the multiple-optical resonator to obtain an optimum magnitude for the mode gain difference.